t/perf/README: elaborate on output format

Signed-off-by: Nipunn Koorapati <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: nipunn1313

t/perf/README

@@ -28,6 +28,8 @@ the tests on the current git repository.
     7810.3: grep --cached, cheap regex       3.07(3.02+0.25)
     7810.4: grep --cached, expensive regex   9.39(30.57+0.24)
 
+Output format is in seconds "Elapsed(User + System)"
+
 You can compare multiple repositories and even git revisions with the
 'run' script: